Modelling of simultaneous heat and mass transfer considering the spatial distribution of air velocity during intermittent microwave convective drying

摘要

Drying is a complex simultaneous heat and mass transfer process where water is migrated from the sample to the drying air through continuous evaporation. The rate of evaporation can be accelerated by the intermittent application of microwave energy with convective drying, which is called intermittent microwave convective drying (IMCD). However, theoretical aspects of IMCD process is not clearly understood yet, which is required to obtain a better -quality product and for designing energy efficient IMCD drying systems. Therefore, several researchers have attempted to develop a theoretical modelling framework for IMCD. However, most of the existing models do not consider the spatial distribution of air velocity around the product, which is important as it significantly affects the heat and mass transfer coefficient throughout the sample and eventually alters the drying kinetics. This paper aims to address this research gap by developing an IMCD modelling framework through integrating a simultaneous heat and mass transfer model with a computational Fluid Dynamics (CFD) model (drying air flow) under the condition of volumetric heating. A 3D model was developed, and the simulated results were validated with the experimental results. It was found that the integration of fluid flow (CFD) with an IMCD model significantly affected the drying kinetics as heat and mass transfer coefficients spatially varied throughout the sample. Drying models may thus overpredict the drying kinetics like moisture content if the spatial distribution of air velocity is not taken into account. It was also found that the nonuniform microwave power absorption caused an inhomogeneous distribution of the sample temperature. The new findings and knowledge from this study will lead to the development of more accurate drying models.